The use of conveyor belts to transport material into a microwave applicator is well known in the art. It is also well known that the leakage of microwave energy out from the oven or applicator and into the surrounding environment is highly undesirable. Any significant amount of leakage can cause injury to persons in the region where the microwave energy escapes, and can cause interference to microwave communication and aviation systems. Therefore, it is necessary in all said applications to provide a reliable means for controlling the microwave energy and to provide safety features which will prevent injury to humans or other equipment in the region.
A conveyor necessarily requires that the microwave application region have openings in order to provide for entrance and exit of the conveyor belt.
The material on the conveyor belt will itself form a waveguide because of its different dielectric constant with respect to the conveyor belt or air which surrounds it. Therefore, the material passing into and out of the application region of a microwave heater becomes a waveguide and forms a path for escape of microwave energy into the surrounding environment.
U.S. Pat. No. 4,570,045 to Jeppson shows the use of a microwave heating chamber wherein the containment walls of the chamber are constructed of a dielectric material. The dielectric material, when used as a containment for the heating chamber, absorbs a significant amount of the energy that would otherwise be available for the heating of materials. In FIG. 3 Jeppson discloses the use of an absorptive material surrounding the conveyor as it enters or exits the heating chamber. This use in the immediate entrance to the heating chamber also consumes substantial amounts of microwave energy that could otherwise be utilized for heating of the product on the conveyor belt. Jeppson does not disclose the use of a reactive element in combination with the absorptive material. Jeppson also fails to disclose any relationship between the substance being heated 16 and the material of the absorptive choke which surrounds the conveyor openings as well as the entire heating chamber.
U.S. Pat. No. 4,488,027 to Dudley et al. (hereinafter Dudley) shows the use of a choke tunnel which has a high impedance to microwave energy at the operating frequency. The first tunnel portion 16 is metallic and operates as a reactive choke which causes the microwave energy to bounce back to the applicator and into the material being treated, and not absorbed by the choke. This section of the Dudley choke conserves energy by redirecting it back to the applicator and into the material being treated. Dudley also points out that this section is primarily efficient at the designed frequency and has little beneficial effect at the harmonic frequencies of the fundamental frequency of the microwave treatment device.
Attached to the metallic tunnel 16 is a second tunnel 18 which is also made of metal but in addition includes a thermomagnetic layer along its inner surfaces which will absorb microwave energy. This second region 18 is used to absorb the microwave radiation that leaks from the first all metallic reactive region.
Neither Dudley nor Jeppson discloses any relationship between the absorptive material and the materials being treated by the microwave applicator or oven. However, if the dielectric constant of the materials being treated is different from that of the surrounding areas, partial or total reflections of microwave power occurs at the interface, which causes the material layer above the conveyor to act as a dielectric waveguide. This waveguide effect confines the microwave within the material layer and allows the microwave to propagate through the absorptive choke without significant attenuation which causes substantial leakage of microwave energy out of the desired containment region and into the environment where humans may be injured, or interference with communications may occur. The Jeppson and Dudley patents fail to disclose any relationship between the dielectric constants of the material being treated and the surrounding material used for the absorptive choke. This is the main reason for the ineffectiveness of Jeppson's and Dudley's choke to prevent leakage.